(First Invention)
There are known fuel cells which include polymer electrolyte fuel cell, phosphoric-acid fuel cell, molten carbonate fuel cell and solid-oxide fuel cell. Of these, polymer electrolyte fuel cell, which is operable at low temperatures and can readily be reduced in size and weight, is aimed at being mounted on such as fuel cell vehicle, stationary cogeneration system and mobile applications. In the polymer electrolyte fuel cell, the top and back surfaces of a polymer electrolyte film for transferring proton are held by a pair of electrodes on which carbon particles having a platinum catalyst supported thereon are immobilized, and thus-obtained membrane/electrode assembly (MEA) is held between separators having gas diffusion layer (carbon paper) and reaction gas supply grooves formed thereon, thereby a unit cell is formed. In general, a plurality of the unit cells are electrically connected in series so as to form a stack. The separator has surface of regular rough, formed thereon, for supplying reaction gas so as to bring a fuel gas (hydrogen gas) or an oxidizer gas (air) into contact with the electrode, and is configured so as to allow the projected portions formed thereon to contact with the surface of the electrode, and to allow the recessed portions to be supplied with the reaction gas. A conventional separator has been made of carbon, but efforts have been also made on those made of metals, in view of realizing cost reduction, downsizing, and weight reduction of the fuel cell.
Adoption of the metal-made separators such as those made of stainless steel may raise a problem below. That is, a polymer electrolyte film of a polymer electrolyte fuel cell contains sulfonic acid group, and needs moisture in order to exhibit ion conductivity. Contact of the moisture with the separator, however, lowers pH due to the sulfonic acid group, and this allows corrosion of the separator to proceed in a power generation environment of the fuel cell. Corrosion of the separator results in deterioration of the polymer electrolyte film due to eluted metal ion, locally raises the electric resistance, and undesirably lowers the output of the fuel cell due to increase in the internal resistance.
Various efforts have therefore been made in view of prevent corrosion of the separator. For example, Japanese Laid-Open Patent Publication “Tokkai” No. 2001-68129, “Tokkai” No. 2000-021418 and “Tokkaihei” No. 10-228914 disclose separators made of stainless steel having an Au plated film of a predetermined thickness formed on the surface thereof. Of these, the above publication “Tokkai” No. 2001-68129 discloses a separator successfully reduced in influences of pinholes in the Au plated film formed on the surface of the separator, and thereby having an improved corrosion resistance, by closing the pinholes by roller pressing or resin molding. On the other hand, the publication “Tokkaihei” No. 10-228914 discloses a separator having an Au plated film of relatively as thin as 0.01 to 0.06 μm partially formed in an area of the metal base composing the separator, to be brought into direct contact with the electrode, and thereby having a reduced contact resistance with the electrode while keeping a desirable level of corrosion resistance.
The separator disclosed in the publication “Tokkai” No. 2001-68129 is, however, fabricated by first forming an under-plated film prior to the Au plating, the Au plated film is then formed to a relatively large thickness in order to reduce the pinholes possibly formed in the Au plated film, and the obtained Au plated film is further roller-pressed, so that this inevitably increases the number of process steps of the fabrication, and also increases Au consumption. Also the separator disclosed in the publication “Tokkaihei” No. 10-228914 is disadvantageous in terms of simplification of the fabrication process, because the Au plated film is partially formed in the portion to be brought into direct contact with the electrode. Moreover, the separator has no Au plated film in the recessed portions (which serves as a gas flow path) which are not brought into contact with the electrode, and this may result in only a limited corrosion resistance against puddles formed in the portions. Both of the separators for fuel cell disclosed in the publications “Tokkai” No. 2001-68129 and “Tokkaihei” No. 10-228914 are fabricated by cutting a metal base to be processed into the separators, and then subjecting them to the Au plating, but this is not convenient in terms of fabrication process, because the individual separated metal bases must be plated.
(Second Invention)
Because a metal separator for polymer electrolyte fuel cell allows an electrode of a unit cell and another electrode of the adjacent unit cell to electrically contact with each other, and separates the reaction gas, it is necessary for the separator to have an excellent electric conductivity, an excellent gas tight performance to the reaction gas, and an excellent corrosion resistance against power generation reaction based on a hydrogen/(oxygen or air) redox system.
There are known conventional metal separators for polymer electrolyte fuel cells such as having a large number of regular rough patterned grooves, for allowing the fuel gas or oxidizer gas to pass through, formed thereon by cutting carbon plates such as those composed of graphite or the like. The fabrication according to this method, however, suffers from increase in costs of the carbon materials and cutting process, and this raises difficulty in practical application of the separators in view of costs. Another problem resides in that the carbon plate cannot be thinned due to its poor strength, and therefore cannot be reduced in size.
There have been developed metal separators for polymer electrolyte fuel cells, composed of a readily-machinable stainless steel having a treated surface. The publication “Tokkaihei” No. 10-228914 discloses use of SUS304 as the stainless steel, and the publication “Tokkai” No. 2000-021418 discloses use of SUS316. The publication “Tokkai” No. 2000-256808 discloses a stainless steel for polymer electrolyte fuel cell containing 30% or less of Cr, and if necessary also at least either of Mo: 10% or less and Ni: 25% or less, and satisfies a relation of 10−0.3×([Cr %]+3×[Mo %]+0.05×[Ni %])≦5, and the balance of mainly Fe.
As materials for composing the conductive separators for polymer-electrolyte-type fuel cells, the publication “Tokkai” No. 2001-243962 discloses a ferritic or an austenitic stainless steel having a carbon content not exceeding 0.03%, a ferritic or an austenitic stainless steel having a carbon content of less than 0.03% and a Mo content of 1.5 to 8%, an austenitic stainless steel having a carbon content of 0.03% or less and a nitrogen content of 0.1 to 0.3%, and so forth.
The above-described SUS304 and SUS316 are, however, known to have problems in the corrosion resistance due to their large contents of C, Mn and S. The stainless steel disclosed in the publication “Tokkai” No: 2001-243962 suffers from a poor machinability and a large cost, due to its substantially large Cr content, and also a large Mo content.
The separators for polymer electrolyte fuel cell is exposed to an extremely corrosive environment of sulfuric acid acidity and steam at about 80° C. or more during power generation. An extremely high corrosion resistance is therefore required for the separators. On the other hand, metal separators workable by plastic working into complicated geometry have also been developed for cost reduction, wherein the above-described corrosion resistance is required also for this sort of separators.
Conventional measure for improving corrosion resistance of the separators relates to use of a metal base plate composed of stainless steel or the like for configuring the separator, having the surface of which covered with a cover film of a noble metal such as Au, Pt or the like by plating and vacuum evaporation showing a higher corrosion resistance than that of the metal base.
In the formation of the cover film of a noble metal, the resultant cover film will have a surface of regular rough conforming to a fine surface of regular rough which intrinsically resides on the surface of the metal base plate, but with an enphasized profile. In recessed portions on the surface of such film, or in crystal grain boundary of the metal base plate, the noble metal film is less depositable as compared with the projected portions or other portions, and the film will therefore tend to be thinned. The recessed portions on the surface of the noble metal film and portions in the vicinity of the crystal grain boundary will therefore show only a limited corrosion resistance, and this makes the corrosion more likely to proceed from these portions towards the inner portion of the separator.
(Third Invention)
The separators for polymer electrolyte fuel cell is exposed to an extremely corrosive environment of sulfuric acid acidity and steam at about 80° C. or more during power generation. An extremely high corrosion resistance is therefore required for the separators. On the other hand, metal separators workable by plastic working into complicated geometry have also been developed for cost reduction, wherein the above-described corrosion resistance is required also for this sort of separators.
Conventional measure for improving corrosion resistance of the separators relates to use of a metal base plate composed of stainless steel or the like for configuring the separator, having the surface of which covered with a cover film of a noble metal such as Au, Pt or the like showing a higher corrosion resistance than that of the base element by plating or vacuum evaporation.
In the formation of the cover film of a noble metal, the resultant cover film will have a surface of regular rough conforming to a fine surface of regular rough which intrinsically resides on the surface of the metal base plate, but with an emphasized profile. In recessed portions on the surface of such film, or in crystal grain boundary of the metal base plate, the noble metal film is less depositable as compared with the projected portions or other portions, and the film will therefore tend to be thinned.
The recessed portions on the surface of the noble metal film and portions in the vicinity of the crystal grain boundary will therefore show only a limited corrosion resistance, and this makes the corrosion more likely to proceed from these portions towards the inner portion of the separator.
(Fourth Invention)
With respect to the metal separator and current collector for fuel cells, there is proposed a technique of forming a thin Au plated film on the surface of a metal base such as stainless steel, in order to keep a desirable level of corrosion resistance and to lower the contact resistance (Japanese Laid-Open Patent Publication “Tokkaihei” No. 10-228914). This sort of treatment is also effective for various materials for electric contacts and terminals. According to the proposal, it is described that those having a Au plated film of 0.01 to 0.06 μm thick directly formed on the stainless base show no Cr elution even after being subjected to nitric acid aeration test (JIS H8621) for one hour, and that this proves formation of no pinholes.
The metal separators in practical polymer electrolyte fuel cell will be exposed to more severe conditions such as a temperature of as high as 100° C., so that higher corrosion resistance enough to prove no elution of metal ions even under more severe test conditions is required, such as under dipping in a boiling sulfuric acid solution of pH 2 for 168 hours. Increase in the thickness of the plated film may substantially solve the problem, but the metal separators for fuel cell, used in a form of stack comprising a large number thereof, is not practical on the cost basis unless the thickness of the plated film is 100 nm or thinner.
The separators having a Au plated film (0.01 to 0.06 μm thick) formed on the surface of the metal base, in particular of the stainless steel base, by the Au plating process described in the aforementioned Japanese Laid-Open Patent Publication “Tokkaihei” No. 10-228914, which specifically conforms to processes of “degreasing-->cleaning-->surface activation-->cleaning-->Au plating-->cleaning-->drying”, however, was found to show elution of elements composing the metal base in the dipping test in a boiling sulfuric acid solution of pH 2 for 168 hours, and the amount of elution thereof was found to considerably vary case by case. It is therefore obvious that a conductive material having an excellent corrosion resistance equivalent to that of the metal separator available for the polymer electrolyte fuel cell cannot be obtained only by subjecting the surface of the metal base to the Au plating according to any publicly-known methods.
Aiming at providing a conductive corrosion-resistant component durable in the above-described severe test, the present inventors made search for reasons for the poor corrosion resistance of the known Au-plated products, and reached the conclusions below:                the surface of the metal base and noble metal film plated thereon contain a larger amount of impurities than expected, and these impair the corrosion resistance of the film;        there is an intermediate layer containing the impurities at least partially between the noble metal film and metal base, and this lowers adhesiveness of the film per se to the metal base; and        the impurities are supposed to be incorporated before foreign films on the surface of the metal base, such as passivation film, oxide film and contamination film, which are harmful to corrosion resistance, are completely removed, and the noble metal film is directly formed thereon.        
A conceptual expression of the above conclusions is given as an upper drawing of FIG. 12. This is not a product of a mere imagination, but is experimentally supported by Auger analysis which is given as a lower drawing of FIG. 12.
Residence of any foreign films and intermediate layer may result in the following nonconformities:                foreign film components produce the pinholes in the noble metal film, which serve as initiation points of corrosion;        any sites of the intermediate layer having a small electric conductivity may vary current density in electroplating, or locally worsens the uniformity, and this may increase the pinholes and reduce denseness of the film; and        a poor adhesiveness between the intermediate layer and noble metal film may readily result in separation of the noble metal film triggered by some external impact.        
It is therefore an object of the first invention to provide a metal component for fuel cell which is satisfactory in the corrosion resistance and allows easy fabrication at low costs, and a method of manufacturing the same, and is further to provide a fuel cell having thus-manufactured metal component for fuel cell.
An object of the second invention is to provide an austenitic stainless steel for polymer electrolyte fuel cell excellent in resistance against sulfuric acid acidity.
An object of the third invention is to provide a highly-corrosion-resistant, polymer electrolyte fuel cell material such as metal separator, current collector and so forth, and a method of manufacturing a polymer electrolyte fuel cell material capable of manufacturing the same in a reliable manner.
An object of the fourth invention is to provide a corrosion-resistant conductive component, in particular metal separator for fuel cell, comprising a metal base and a noble metal film formed thereon, overcoming the aforementioned problems, being very few in the pinholes, dense in the film quality, excellent in the adhesiveness to the metal base, and therefore being durable against severe conditions of use.